The present invention relates to cathode-ray tube circuits in general, and in particular to a dynamic focus and astigmatism correction circuit for cathode-ray tubes having electromagnetic or electrostatic deflection systems.
The focusing of an electron beam by electric or magnetic fields in a cathode-ray tube, hereinafter abbreviated CRT, is closely analogous to the focusing of light rays in an optical system. Not surprisingly, the electro-optical system of a CRT is plagued by distortions very similar to those which plague optical systems.
Two of the more troublesome distortions are defocusing and astigmatism. Astigmatism is a focal defect in which the electrons in different axial planes come to focus at different points. Under these conditions the CRT spot is not round, thus causing different trace widths depending upon the direction of the trace. More information concerning the various CRT distortions may be found in The Cathode-Ray Tube, by G. Parr and O. H. Davie, Reinhold, 1959, which is herein incorporated by reference. Numerous schemes have been developed to dynamically correct for the CRT distortions.
The article "Correction of Deflection-Abberations by Analog Computer" by K. Schlesinger and R. A. Wagner in Vol. 112, No. 8, IEEE Transactions on Electron Devices, which is herein incorporated by reference, describes a system for dynamic control of focus and astigmatism. A complex analog computer containing special-purpose computing tubes is used to generate the signals required to correct the display. One of the signals generated corrects for corner astigmatism. Because of variations in the CRT screen, it is desirable to have separate astigmatism correction for each corner of the screen. The circuit of Schlesinger and Wagner, however, contains no provision for such individual adjustment.